Ultrasonic images formed from harmonics generated from nonlinear propagation without the use of contrast agents have emerged within the medical ultrasound field as a valuable new diagnostic mode in medical imaging. A preferred tissue harmonic image is formed after listening for a second harmonic of the transmitted fundamental signals. The use of injected nonlinear contrast agents can also be used to further increase the signals from frequency bands other than the transmitted fundamental frequencies, such as the second harmonic of the fundamental or half the fundamental frequency, a subharmonic. Over the last few years there has been an increased interest in using injected nonlinear contrast agents for enhancing the diagnostic examination. More recently, the use of low amplitude excitation for the purposes of minimizing agent disruption, improving contrast between agent and tissue, and visualizing small blood vessels has generated increased interest.
Reduced clutter noise, reduced reverberation artifacts, and improved contrast have made tissue harmonic images preferred in difficult to image patients. Since the amount of tissue-generated harmonics can be substantially less than the fundamental, since harmonic energy is gradually accumulated from the face of the transducer, and since tissue attenuation is greater at higher frequencies, images generated from harmonic energy suffer from an inferior signal-to-noise ratio (SNR) as compared to images formed from fundamental energy alone. Hence, harmonic images can lack critical diagnostic information at shallow areas near the transducer face, at deep areas near the penetration limit of fundamental images, at outer edges of scan formats, and at large steering angles for individual ultrasound lines and individual transducer elements. Increased transmit voltages can increase returned signal levels and often SNR, but a maximum is reached based on practical electrical limits.
Imaging with low amplitude excitation while using injected contrast agents can minimize agent disruption, such as agent destruction, leaving more agent available for longer examination times and increased detection of small vessel flow. However, low amplitude excitation can produce unacceptably poor image quality where agent is not present or present in low concentrations. Further, contrast agent signals returns in the second harmonic frequency band and other bands such as the subharmonic and ultraharmonic frequencies may be limited by conventional transmit pulses. Since transmit voltages can not be increased without disrupting contrast agents, images with low amplitude excitation will exhibit poor SNR.
Frequency modulated (FM) pulse-compression is a well known technique for increasing the average power of a signal without increasing the instantaneous peak power. This technique was developed for radar applications in the 1940's and 1950's and more recently suggested in the medical ultrasound field for fundamental imaging (M. O'Donnell, Coded Excitation System for Improving the Penetration of Real-Time Phased Afray Imaging Systems, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 39, No. 3, pp. 341-51, May 1992); and contrast agent imaging (Y. Takeuchi, Coded Excitation for Harmonic Imaging, Ultrasonics, PH-3, 1996.).